The present invention relates in general to substrate manufacturing technologies, and in particular to methods for reducing photoresist distortion while etching a substrate in a plasma processing system.
In semiconductor fabrication, devices such as component transistors may be formed on a substrate, e.g., a semiconductor wafer or a glass panel. Above the substrate, there may be disposed a plurality of layers from which the devices may be fabricated. To facilitate understanding, the discussion that follows focuses on oxide etching in which a wafer having thereon a photoresist mask and an oxide layer disposed thereunder is etched in a plasma etcher.
Using plasma generated from an etchant source gas that includes oxygen, argon, and fluorocarbon and/or hydro-fluorocarbon, areas of the oxide layer that are unprotected by the mask are etched away, leaving behind vias, contacts, and/or trenches that eventually form electrical structures on the substrate.
Generally speaking, irrespective of the composition of the etchant source gas, the generated plasma typically includes, besides molecules and radicals, ions having energy in the millielectronvolt (meV) range and electrons having energy in the electron volt (eV) range. During the plasma etch process, as a consequence of the ion bombardment (lower energy but larger mass) and/or electron bombardment (lower mass but considerably higher energy), the photoresist mask may experience distortion that is commonly called wiggling.
The distortion may affect just the top surface of the photoresist or it can be more extensive, affecting the vertical sidewalls of the photoresist. Once the distortion starts, the severity of the distortion tends to increase as the etch progresses. Also, depending on the chemistry employed during the etch process, the degree of photoresist distortion may vary.
To facilitate discussion, FIG. 1A illustrates a simplified cross-sectional view of a layer stack 100, representing the layers of an exemplary semiconductor IC prior to a lithographic step. In the discussions that follow, terms such as “above” and “below,” which may be employed herein to discuss the spatial relationship among the layers, may, but need not always, denote a direct contact between the layers involved. It should be noted that other additional layers above, below, or between the layers shown may be present. Further, not all of the shown layers need necessarily be present and some or all may be substituted by other different layers.
At the bottom of layer stack 100, there is shown a silicon dioxide layer 108, typically comprising SiO2. Above the silicon dioxide layer 108, there is disposed an overlaying photoresist layer 102.
Photoresist layer 102 is commonly patterned for etching through exposure to light, such as ultra-violet light. By way of example, one such photoresist technique involves the patterning of photoresist layer 102 by exposing the photoresist material in a contact or stepper lithography system to form a mask that facilitates subsequent etching.
For illustration purposes, FIG. 1B shows an idealized cross-sectional view of layer stack 100 of FIG. 1A after photoresist layer 102 has been formed via the lithography step. In this example, photoresist has been removed to form a photoresist trench 112, leaving two columns of photoresist 102. Since modern IC circuits are scaled with increasingly narrower design rules to achieve greater circuit density, feature sizes (i.e., the cross-section dimensions 110 of the vias, trenches, or contacts) have steadily decreased.
FIG. 2 shows the cross-sectional view of a layer stack 220 in which photoresist layer 202 has been distorted during the etch. As seen in FIG. 2, horizontal surface 210 and vertical surface 212 have been substantially distorted during the plasma etch. The asymmetric polymer deposition on the sides of the photoresist mask 212 and the asymmetric photoresist faceting 218 are the main factors that accompany the distortion of the photoresist mask. As a consequence of the distortion, the photoresist mask may not have sufficient strength to withstand further plasma bombardment, and consequently, the photoresist columns may fall over, partially or totally covering the opening of the feature to be etched 214. Accordingly, the distortion may cause defects in the resultant etch features, leading to a lower percentage yield.
In particular, it has been observed that the distorted photoresist exhibits wiggling, or wave-like patterns in the columns of the photoresist material when viewed from the top of the substrate. To illustrate, FIG. 3 shows a top view of a distorted photoresist mask 320 in which the plasma etching process has created wiggles 302 in the photoresist layer. The resulting mask pattern can partially or completely block the intended removal of the substrate material. The photoresist mask 320 was originally patterned to form rectangular features. However, distortion of the photoresist causes irregularly shaped features 312, as shown.
If the photoresist mask suffers from excessive wiggling, striations can occur. Striations may be caused when photoresist wiggling exposes certain, irregular faceted photoresist portions of the photoresist columns to the vertical etch component, where such photoresist portions are normally protected from the vertical etch component had there been no photoresist wiggling. With reference to FIG. 4A, the exposed photoresist portions are shown by shaded portions 406, which are exposed to the vertical etch component 410 after photoresist wiggling causes the photoresist columns 402 to bend over.
The result of etch process is shown in FIG. 4B in which underlying oxide regions 408 are undesirably removed by the etch process, causing etch features 404 to have an irregular shape, different than the intended. Thus, the feature may have an enlarged distorted sidewall 414 instead of the intended sidewall 412. The striations are shown in exemplary FIG. 4C as undesirable sharp “vertical trenches” 452 in the otherwise smooth vertical sidewalls of the vias 454. Such striation in the etched features alters the intended electrical and functional characteristics of the resultant device, leading to defects in the resultant device.
Additionally or alternatively, if the photoresist wiggles converge and protrude inside the intended outline of the mask hole (as viewed from the top of the substrate), the resulting distorted mask pattern can partially or completely block the intended removal of substrate material. These randomly affected vias represent defects in the resulting device and undesirable lowering yield. FIG. 5 is a cross sectional view of an etched feature 504 in a substrate 508, where the photoresist wiggles have caused the photoresist 502 to form a partial block 520, which partially blocks the etching of the feature 504 in the substrate 508.
FIG. 6 is a cross-sectional view of a substrate with a photoresist mask 602, where wiggling of the photoresist mask 602 has caused the photoresist to create a complete block 606 over a feature 604. The complete block 606 of the photoresist 602, stops the etching of the feature, so that the feature 604 is only partially etched in the substrate 608, as shown.